Laser flash photolysis (LFP) is a technique utilized to study reaction mechanisms in chemical and biological processes. The technique was introduced in 1966 by Lindqvist at the CNRS in France and the technique was quickly developed by various research groups around the world. LFP was brought about by the invention of the laser in the early 1960s. The technique of LFP consists of a pulsed laser source that generates a chemical species in a sample to be studied, an optical and electronic system capable of sensing optical changes in a sample, and a computer suitably equipped to selectively capture, process, and display the data. The optical and electronic systems constitute a fast spectrometer capable of acquiring spectra of short-lived chemical species called “intermediates”. The optical and electronic systems then record the evolution of the intermediates over time. The time resolution in such fast spectrometer can be achieved by two primary methods.
A first method includes use of fast electronics where a readout of a fast detector is digitized and recorded in real time, or when an electronic gating is applied to the detector. The electronic gating is typically used with array-based spectrometers where the output cannot be processed rapidly enough to perform real time data acquisition. Both techniques typically utilize continuous wave (CW) or pulsed xenon arc lamps as a probe light source. Due to the low intrinsic brightness and poor collimation of a probe beam produced by the probe light source, an optical overlap between the probe and a pump (excitation) beam takes place over an area of approximately 1 cm2, thereby placing energy requirements on the laser pulse necessary to induce chemical changes in the sample. The corresponding pump laser pulses typically have energy of a few millijoules. Because of the pulse energy requirement, only a limited number of lasers, known as Q-switched lasers, can be used with the xenon arc lamp probe light source to produce the required energy.
A second method is called optical gating or the “pump-probe” method. In this method, the dynamics of a chemical change of a sample is monitored by studying a series of light pulses from a laser at different times as the light pulses (pump beam) are passed through the sample. The probe and pump beams travel through the same volume of the sample studied. A pulse of the pump beam induces a transient chemical change in the sample which affects the optical properties of the sample. A spectrum of a pulse of the probe beam passing through the sample is altered by the changes made to the sample by the pump beam depending on when the probe pulse arrives at the sample with respect to the pump pulse.
Where the probe beam travels in front of the pump beam, the probe beam will only measure the sample before the excitation event. As the probe beam is delayed, it arrives at the sample simultaneously with the pump pulse, corresponding to a time zero. The delay of the probe beam is incrementally increased over a desired time interval. The corresponding changes in the probe beam monitored by a detector are therefore assigned to particular delays (time) after the excitation event. A series of probe beams at various delays represents information about the dynamics of the changes in the sample caused by the pump beam.
At each of the delays of the probe beam, two spectra of the probe beam are recorded by the detector, A first spectrum corresponds to the probe beam traveling through the sample together with the pump beam. A second spectrum, a reference spectrum, corresponds to the probe beam sent through the sample without the pump beam. Usually at a particular pump probe delay, a series of such probe spectrum pairs are averaged in order to obtain a sufficient signal to noise ratio. The pump beam energy in such experimental setups is often limited to several microjoules. Therefore, in order to achieve comparable instrument sensitivity and similar photon flux in the excitation beam, the pump beam and the probe beam are spatially overlapped in the sample over an area less than 1 mm2. Generation of a probe beam that can satisfy the above requirement is possible only if a highly collimated beam such as a laser is used.
Optical gating has been successfully used with femtosecond and picosecond lasers. The femtosecond or picosecond laser output is split into several parts, one of which is used to produce a probe beam with desired wavelength specifications, usually through super-continuum generation or optical parametric amplification. The materials used for super-continuum generation are typically bulk materials—crystals such as sapphire, calcium fluoride, etc. or liquids such as water, etc. The resulting beam is then used to probe the photo-induced changes in the sample. The time resolution is realized by varying the travel path length of the probe beam with respect to the pump beam, which allows for extremely high temporal resolution, down to several femtoseconds. However, in order to generate a super-continuum in bulk materials such as sapphire one needs to have laser pulses with high peak power (MegaW), which can be produced by only a limited number of lasers including amplified femtosecond lasers. Such amplified femtosecond lasers are expensive and have a large footprint (8-10 ft2).
Commonly owned U.S. Pat. No. 7,817,270 B2 shows a nanosecond pump-probe LFP system that is adapted to a substantially lower energy requirement of a pump light source and a probe light source. The LFP system includes a photonic crystal fiber based probe light source, a pump light source adapted to produce light pulses with nanojoule or higher energy, a delay generator, a computer, and a detector.